1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and apparatus for transmitting data of a radio link control (RLC) layer.
2. Description of the Related Art
The Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system using a wideband Code Division Multiple Access (CDMA) scheme based on Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
The Third-Generation Partnership Project (3GPP), responsible for the UMTS standardization, is currently discussing Long Term Evolution (LTE) as the next generation UMTS system. The LTE is targeted for commercialization around 2010 and is a technology for implementing high-speed packet based communication at about 100 Mbps. To this end, various plans are under discussion, including a plan to reduce the number of nodes located on a communication path by simplifying a network architecture, a plan to approximate wireless protocols to a radio channel as close as possible, and so forth.
FIG. 1 is a block diagram illustrating the configuration of an evolved UMTS mobile communication system.
Evolved UMTS Radio Access Networks (“E-UTRANs” or “E-RANs”) 110 and 112 are simplified into 2-node structures of Evolved Node Bs (“ENBs” or “Node B”) 120, 122, 124, 126, and 128 and, anchor nodes 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 over the E-RANs 110 and 112.
The ENBs 120 to 128 correspond to the existing Node Bs of the UMTS system and connect to the UE 101 through a radio channel. In comparison with the existing Node Bs, the ENBs 120 to 128 perform more complex functions. Because all user traffic, as well as a real-time service, such as Voice over IP (VoIP) using an IP, are transmitted through a Shared CHannel (SCH) in the LTE system, a device capable of collecting situation information of UEs and performing a scheduling process is required. The ENBs 120 to 128 are responsible for the collecting and scheduling processes.
The LTE system performs a Hybrid Automatic Repeat Request (HARQ) between the ENBs 120 to 128 and the UE 101 as in High Speed Downlink Packet Access (HSDPA) and Enhanced uplink Dedicated CHannel (E-DCH). However, because various Quality of Service (QoS) requirements cannot be satisfied only by the HARQ scheme, an outer Automatic Repeat request (ARQ) may be performed in an upper layer. The outer ARQ is also performed between the UE 101 and the ENBs 120 to 128.
To implement a transmission rate of a maximum of 100 Mbps, the LTE system may employ a wireless access technology of an Orthogonal Frequency Division Multiplexing (OFDM) scheme at a system bandwidth of 20 MHz. An Adaptive Modulation & Coding (AMC) scheme for setting a modulation scheme and a channel coding rate according to a channel state of the UE may be employed in the LTE system.
Many next-generation mobile communication systems as well as the LTE system use the HARQ scheme as an error correction scheme. HARQ is a scheme which increases a reception success rate by soft-combining previously received data with retransmitted data, without discarding the previously received data. In more detail, an HARQ receiving side determines if there is an error in received data, and transmits a positive Acknowledgement (HARQ ACK) signal for the HARQ or negative Acknowledgement (HARQ NACK) signal for the HARQ to an HARQ transmitting side. The HARQ transmitting side either retransmits the HARQ data or transmits new HARQ data according to the HARQ ACK/NACK signal. Then, the HARQ receiving side soft-combines the retransmitted data with previously received data, thereby reducing an error occurrence rate.
FIG. 2 is a view illustrating the protocol stacks of a next-generation mobile terminal.
Each packet data convergence protocol (PDCP) layer 205 and 240 functions to compress/decompress an Internet Protocol (IP) header. Each Radio Link Control (RLC) layer 210 and 235 performs an operation of resizing RLC service data units (RLC SDUs) into RLC Protocol Data Units (PDUs) having an appropriate size and operates as an ARQ device performing an ARQ operation with respect to the RLC PDUs. Data input to a specific protocol entity from an upper layer is referred to as an SDU of the specific protocol. As shown in FIG. 2, the PDCP layers 205 and 240 are located in a UE and an upper layer, and the RLC layers 210 and 235 are located in the UE and an ENB.
Medium Access Control (MAC) layers 215 and 230 are connected to a plurality of RLC entities configured in one UE, multiplex RLC PDUs from each RLC entity into a MAC PDU, and demultiplex a MAC PDU from a lower layer into RLC PDUs. Data output from a specific protocol entity is referred to as a PDU.
Each PHYsical layer (PHY) 220 and 225 generates OFDM symbols by performing channel-coding and modulation operations with respect to upper-layer data, and transmits the OFDM symbols through a radio channel. Also, each physical layer 220 and 225 transmits OFDM symbols received through a radio channel to an upper layer, after demodulating and channel-decoding the OFDM symbols. Most HARQ operations, including the operations of performing a channel decoding with respect to received data, soft-combining the channel-decoded data with previously received data, and performing a Cyclic Redundancy Check (CRC) operation with respect to soft-combined data, are performed by the physical layers 220 and 225, and are controlled by the MAC layers 215 and 230.
As described above, the RLC layers 210 and 235 ensure reliable data transmission/reception through the ARQ process.
FIG. 3 is a block diagram illustrating a data transmission/reception and retransmission process of an RLC layer.
A transmission buffer 305 of a transmitting-side RLC layer stores RLC SDUs 310 provided from a transmitting-side PDCP layer until the RLC SDUs 310 are transmitted to a receiving-side RLC layer. A framing unit 315 re-configures the RLC SDUs 310 to RLC PDUs 325 having an appropriate length, and transmits the RLC PDUs 325 to the receiving-side RLC layer after attaching an ascending serial number to each RLC PDU. In this case, the RLC SDUs 310 are buffered in a retransmission buffer 320 until an ACKnowledgement (ACK) signal is received from the receiving-side RLC layer. The receiving-side RLC layer stores received RLC PDUs, detects an RLC PDU lost during the transmission by checking the serial numbers of the stored RLC PDUs, and requests the transmitting-side RLC layer to retransmit the lost RLC PDU. RLC PDUs reordered in a reception buffer 330 are assembled into RLC SDUs through a re-assembling unit 335, and are then transferred to an upper layer.
According to the example shown in FIG. 3, while RLC PDU [7] to RLC PDU [10] have been transmitted from the transmitting-side RLC layer, only RLC PDU [7] and RLC PDU [9] have been received by the receiving-side RLC layer and have been stored in the reception buffer 330. The receiving-side RLC layer transmits a status report 340 containing ACK and/or non-acknowledgement (NACK) information, which informs that the RLC PDU [7] and RLC PDU [9] have been correctly received, but RLC PDU [8] has not been received, to the transmitting-side RLC layer. Then, the transmitting-side RLC layer retransmits the RLC PDU [8], which has been stored in the transmission buffer 320, based on the status report 340, and discards the RLC PDU [7] and RLC PDU [9].
When there is a missing RLC PDU, the status report 340 may be transmitted either based on a determination of the receiving-side RLC layer itself or by a request of the transmitting-side RLC layer. A transmission request for a status report issued by the transmitting-side RLC layer is referred to as a “polling.”
FIG. 4 is a view illustrating an example of a polling trigger of an RLC layer and a status report transmission process.
When a transmitting-side RLC layer 405 needs to check an RLC PDU reception state of a receiving-side RLC layer 410 while transmitting RLC PDUs 415 to the receiving-side RLC layer 410, the transmitting-side RLC layer 405 triggers a polling process, which is referred to as a “polling trigger” 417. The polling trigger represents setting a polling bit “p” in an RLC PDU 420 to be transmitted next. When the receiving-side RLC layer 410 has received the RLC PDU 420 including the set polling bit, the receiving-side RLC layer 410 triggers transmission of a status report (see reference number 423). Then, a status report 425 including ACK/NACK information representing a reception status of a reception buffer is configured, and is transmitted to the transmitting-side RLC layer.
A transmission buffer of the transmitting-side RLC layer stores RLC SDUs transferred from an upper layer. The transmitting-side RLC layer supports a discarding function in order to prevent the transmission buffer from overflowing. The discarding function may be implemented by a method using a timer operation, or may be implemented by a method of setting a maximum number of times. However, according to the conventional function of discarding for RLC SDUs, even though there is an RLC PDU for which an ACK signal has not been received from a receiving-side RLC layer to a transmitting-side RLC layer, the RLC PDU may be discarded from the transmission buffer of the transmitting-side RLC layer, thereby causing a failure of data transmission.